This invention relates in general to a pile driver cushion, and more particularly, to a pile driver cushion fabricated from a flexible strip of compressed knitted wire mesh.
Piles are long slender members usually made from trimmed timber, steel or reinforced concrete which are driven downward and into the ground or seabed by means of an apparatus called a pile driver or pile hammer, to support vertical loads such as large buildings, bridges or oil drilling platforms.
The pile driver utilizes a falling weight or ram which is operated by steam, compressed air or by diesel. The ram is not allowed to directly strike the top of the pile as this would cause mushrooming in the case of steel piles or chipping and spalling in the case of concrete piles. To prevent this problem, a steel case with a horizontal partition, known as a helmet, is placed on top of the pile. The underside of the helmet is shaped to conform to the dimensions or configuration of the top of the pile. The top of the helmet has a circular cavity in which is placed a cushion of resilient material. This material acts as a shock absorber and blunts the sharp force loading on the pile at the moment of impact. It also prevents damage to the pile driver when the pile reaches refusal which is generally defined as the point where pile driving resistance exceeds either 300 blows of the hammer per foot for five consecutive feet or 800 blows for one foot when the pile weight does not exceed four times the weight of the hammer.
Cushion materials in current usage fall into three groups; (1) end grain hickory, oak or similar hardwoods, (2) circular pads 1/4 inch to 1/2 inch thick fabricated from asbestos with certain fillers, and (3) round plates of aluminum alternating with disks made from medium weave cotton impregnated with a phenolic resin compound.
When energy is put at a high rate into cushions made from the above materials, their temperature climbs rapidly causing a reduction in the efficiency of driving and contributing to the eventual deterioration of the cushion. Hardwood cushions frequently catch on fire when temperatures above their ignition point are reached. Asbestos based cushions do not catch fire. However, they compact down into a solid rock like mass that has no resiliency or cushioning effect and have to be replaced at frequent intervals. The phenolic resin disks deteriorate through their inability to dissipate heat and while the alternating aluminum disks help dissipate heat somewhat, they do not contribute to the resiliency of the assembly.
There is thus a clear and heretofore unsolved problem of improving the shock absorbing material used in a pile driving operation.
Metal shock absorbing elements of stainless steel wire have been used for vibration damping in applications where vibration and shock are at relatively low levels. For example, in drilling for oil, the weight of the drilling apparatus is transmitted to the drilling bit producing predominantly vibration loading and to a lesser extent, impact loading on the drilling bit during the drilling operation. The tubular drilling string normally includes an anti-vibration and shock tool (known as a "Shock or Damping Sub") positioned immediately above the drill bit for smoothing out the small scale vibration and impact loads associated with the drilling operation. The shock sub includes a stainless steel wire knitted into a cloth like mesh and compressed in a compression die into an annular ring. The annular ring is positioned around the shaft of the shock sub in such a manner as to receive the axial loading on the drill bit.
The annular ring is fabricated in an annular ringshaped die of fixed volume by isometric compression. A tubular stocking of knitted wire mesh is rolled upon itself from each end to form a double doughnut of approximately 41/2-51/2" in outside diameter. The double doughnut is placed within the annular cavity of the compression die and is subsequently compressed with appropriate force to compact the double doughnut into a uniform annular ring structure. This ring structure is found to be of uniform resiliency since the problem of material flow which is known to occur in larger volume die cavities is not significant in a compression die of relatively small volume.
The sharp impact loads which occur during a pile driving operation are at least ten times greater than the vibrational and impact loads encountered in a well drilling operation where the predominant force is one of vibration.
Thus a wire mesh annular ring constructed for absorbing forces occuring in a drilling operation will not absorb the forces which occur during a pile driving operation at a steady rate of about 50-60 blows per minute. To be effective to protect the pile driver and the pile during the steady force loading which occurs during the pile driver operation, a cushion of knitted wire mesh must have uniform resiliency and must retain that resiliency during the entire pile driving operation, conditions which knitted wire mesh heretofore have not been able to obtain.
Cushions for use in a pile driver operation are fabricated in sizes ranging from 6" to 72" in diameter. The fabrication of cushions from knitted wire mesh over this range of diameters cannot be successfully or economically fabricated in one piece in a compression die. It is contemplated, that if a tubular stocking of knitted wire mesh were placed within a large volume die cavity and compressed using isometric compression to form a cushion in a manner similar to that used to form the annular ring used in the vibration tool, the problem of material flow within the die cavity would result in the cusion being of non-uniform resiliency. Further, to fabricate cushions having different diameters would require a compression die for each cushion of desired diameter which is generally impractical from an economic consideration.
It is also expected that cushions fabricated in a compression die would not be capable of achieving a density sufficiently high to impart the resiliency required for use in a pile driving operation. This occurs due to the compression forces being distributed over the total surface area of the cushion which is far in excess of the surface area of the annular ring and thus reducing significantly the compression forces available for compacting the knitted wire mesh. The compression forces that are required to compact to a high density a layer of knitted wire mesh having a diameter of 6" is greater than what is economically practical for conventional presses. The compression force requirements increase significantly with increasing diameter of the cushion and accordingly presses to fabricate cushions having increasing diameters greater than 6" are heretofore unavailable.
The use of a pile driver cushion of non-uniform resiliency and low density in a pile driving operation would result in the cushion being rapidly further compressed under the action of the pile driver to a point where the resiliency of the cushion would be insufficient to protect either the pile driver or the pile from damage.